Process and apparatus for drying lumber



June 21,1932. F, w R N 1,863,943

PROCESS AND APPARATUS FOR DRYING LUMBER Filed Dec. 7, 1928 2Sheets-Sheet 1 INVENTOR.

ATTORNEYJ BY I June; 21, 1932. w, Ru m 1,863,943

PROCESS AND APPARATUS FOR DRYING LUMBER Filea Dec. 7, 1928 2Sheets-Sheet 2 A TTORNEY3.

Patented June 21, 1932 UNITED STATES PATENT OFFICE FREDERICK W. RUBIN,OF INDIANAPOLIS, INDIANA, ASSIGNOR, BY MESNE ASSIGN- IVIENTS, TO THEUTILITY MANUFACTURING AND SALES COMPANY, OF INDIAN- APOLIS, INDIANArnocnss AND APPARATUS ron DRYING LUMBER Application filed December 7,1928. Serial No. 324,447.

method heretofore employed consistedv in.

maintaining a substantially constant relative humidity andintermittently steaming the lumber by discharging steam therein whenthere appeared tobe danger of the lumber checking during the dryingoperation. This intermittent steaming which occurred at intervalsthroughout the drying process had the effect of materially raising thevrelative humidity at these periods which was contrary to the fundamentalprinciple involved in the process of maintaining the relative humiditysubstantially constant. It was also the practice to endeavor to maintaina substantially constant relative humidity, and gradually decreasing thesame withthedecrease of moisture in the lumber, but maintaining asubstantially constant dry and wet bulb tem perature, which practice wasnot entirely satisfactory for rapiddrying.

It is the object of this invention to improve upon the art of dryinglumber wherein the practice of intermittent steaming or the alternateraising and lowering of the relative humidity between wide limits iseliminated, as well as the practice of maintaining a substantiallyconstant relative humidity by substantially constant wet and dry bulbtemperatures.

This invention contemplates, on the other hand, maintaining for eachcycle a substantially constant or unvarying relative humidity by thealternate and simultaneous ra1s1ng and lowering of the dry and wet bulbtemconditions, would theoretically maintain an absolute constantrelative humidity, the lumber is subjected to rapid variations in' bothdry heat and steam application which acts to highly'heat the lumberwhile safe-guarding the surface against checking, and then rapidlyreducing the heat of the air surrounding the surface ofthe lumber. Thus,the interior of the lumber is highly heated by the abnormally hightemperatures periodically reached, whereupon the moisture containedinteriorly is caused to expand and is forced out to the surface of thelumber, due to the softened condition of the surface maintained by thecorresponding periodic applications of steam.

The invention will be more clearly under stood from the drawings and thefollowin description and claims:

Fig. is a diagrammatic illustration of one means of controlling the wetand dry bulb temperatures. Fig. 2 is an illustrative chart showingthecorresponding periodic rise and fall of the dry and wet bulbtemperatures, with the corresponding substantially constant relativehumidity resulting therefrom.

The invention consists in the process or method of drying lumber byperiodically and simultaneously turning on and 011' both the dry and wetsteam., Thus, as illustrated in the accompanying chart, Fig. 2, the dryand wet steam is turned on for thirty minutes, thussimultaneouslyraising the dry temperature withthe wet temperature. Thereupon, both thedry and wet steam are simultaneously turned off and remain turned offfor 1 hours, permitting at first a rapid reduction in both wet and drytemperature and then a more gradual reduction, owing to the eliminationof heat and moisture from the lumber. Thereupon, the wet and dry steamis again turned on and the cycle is repeated.

Theobject of the operation is to maintain a substantially constantrelative humidity for each cycle by reason of the substantially constantrelation between the wet and dry bulb temperatures. However, it is to beunderstood that it is immaterial to the process or to the drying of thelumber that the relative humidity be constantly maintained at anyparticular percentage so long as it is maintained sufliciently high toavoid any danger of. checking or case hardening in the drying of thelumber. 'As is well known, the actual con- 7 stant relative humiditymaintained will gradually decrease through the drying operation and varyin various parts of the kiln depending largely upon the construction ofthe kiln, the dampers and ventilation thereof, the character of thelumber being dried and the moisture content thereof.

While a clock mechanism or timing device is herein illustrated ascontrolling the operation, whereby the wet and dry steam is turned onduring a definite period and turned off during a definite period, thedrying process may also be accomplished by employing a wet and dry bulbcontrolled mechanism such as disclosed in my Letters Patent No.1,662,283, issued March 13, 1928. Thus, an electric contact for the wetbulb may be set to'turn on both the wet and dry steam when thetemperature reaches a minimum and turn them off when a maximumtemperature is reached, or the dry bulb control may be similarly set toturn on and off both the dry and wet steam when the temperature reachesa minimum or maximum, respectively. In other words, the operation may beconducted by either a clock controlled apparatus, or by an. electricallyor mechanically wet and dry bulb temperature controlled apparatus.

The apparatus disclosed herein embodies one form of control asillustrated in Fig. 1, wherein there is provided a time switch in theform of a clock having a-plurality of minute and hour contact points,the minute contact points being connected individually with the minutecontact switch 11 and the hour contacts with the contact switch 12. Thecontact switches 11 and 12 through manual control thereof or adjustmentwill close a circuit through the solenoids 13 upon the contact of theclock hands with the corresponding contacts in the time switch 10. Theswitch 12 isconnected with the opening solenoid 13 and the switch 11with the closing solenoid 13, said solenoids being connected with thewet and dry steam valves '14 and 15 r for opening or closing the same.Thus when suitably adjusted by manually setting the contact switches 11and 12, when the hour hand reaches the contacts 2, 4, 6, 8, 10 or 12,for purposes of illustration, the open solenoid will be actuated to openthe steam valves and allow bothwet and dry steam as a decasing anddrying medium, respectively, to be simultaneously discharged into thekiln. When the minute hand reaches 30, or thirty minutes later, theswitch 11 being set accordingly, the closing solenoid, willbe energizedfor closing the valves and simultaneously shutting off both the wet anddry steam, thereby permitting the high temperature of the kiln torapidly dry the lumber while the lumber is fully protected by thesimultanefous bath of wet steam, the relative humidity, however,remaining substantially constant.

During this period of'1 hours,the cooling process takes place by thedrop in both dry and wet bulb temperatures, and simultaneously therewiththe moisture of the lumber is carried away from the surface thereofwhich effects a cooling action by reason of the relatively rapidevaporation. The heat which has permeated to the center of the lumberduring the high temperature period expands the moisture in the centerand drives it to the surface where it will be ultimately evaporatedtherefrom. Thus, by the rapid drop in temperature and moisture causingthe evaporation and cooling of the surface of the lumber throughoutrepeated cycles, the interior moisture of the lumber is caused to flowoutwardly therefrom. It will be noted that the periodic baths of steam,however, eliminateany danger of surface checking or case hardening so asto maintain the surface of the lumber always open to permit theexpulsion of moisture from the interior.

Obviously, the durations of time of the respective cycles are dependentupon not only the character and construction of the kiln and the dampersused, but the character and dimensions of the lumber being dried. Thequantity and pressure of the steam is also a determining factor. Whereinthe control is obtainedthrough the medium of wet and dry bulbs placedwithin the kiln, the high and low limits of the wet and dry bulbthermometer temperatures must be determined by the above conditions. I

As illustrated in Fig. 2, there is shown a rapid rise in wet and drybulb temperature during the first twenty minute period. During the.following. tenminute period the rise is less rapid, owing to theabsorption of the heat and moisture by the lumber. At the end of thirtyminutes, when the valves are closed there is a rapid decline in bothtemperatures for thirty-five minutes, the decline being less rapidduring the next twenty-five minutes, owing to the fact that moisture andtemperature is being given off by the lumber. As this chart was derivedfrom an actual application of the process, it will be noted that whiletherise and fall of the dry and wet bulb temperatures are substantiallythe same, there will be slight variations owing to internal conditionsof the kiln including drafts and lumber conditions. As shown at thebottom of the chart, the relative humidity has been 7 calculated for thepurpose of illustrating the factthat it remains substantially constantfor the cycles shown and has no direct hearing one way or the other uponthe process. This results inan irregular wavy line, due to the abovevariations created by conditions of the lumber and the interioratmospheric conditions of the kiln.

The term wet steam as used herein indicates the steam spray, wherein thesteam is sprayed directly into the interior of the kiln carrying themoisture therewith, and, therefore, will affect the wet bulbtemperature. The term dry steam means the dry heat produced by the steampassing through heating COIlS.

Both of the mains leading to the steam spray for delivering wet steam,and into the heating coils for delivering dry steam, are provided with apressure regulating valve 16 of the usual character connected by a pilotline 17 with the pressure gauge 18. Hand valves 19 are also positionedin the mains for manual control. By means of this arrangement, the steampressure in both mains may be maintained substantially equal or constantso as to properly control the operation.

As will be understood in the art, while the relative humidity will bemaintained subst antially constant throughout a cycle of the operation,there will necessarily be gradual decrease in the general trend of therelative humidity from the beginning of the drying process to the endthereof, but said gradual decrease will be substantially unvarying.

It may be noted that the underlying principle involved in the abovedescribed process results in forcing the moisture from the interior ofthe lumber toward the surface thereof by what ma btermed a differentialpressure created by the difference of temperature between the interiorof the lumber and the surface during that portion of the cycle whereinthe temperature is rapidly decreased with the corresponding decreasingin wet temperature, and while the relative humidity remainssubstantially constant. This is caused by the temperature of the surfacedecreasing more rapidly than the interior of the lumber, finallyarriving at an equilibrium of temperature between the interior and thesur face at the end of the cycle when the wet and dry temperatures areagain rapidly raised.

It may further be noted that the moisture absorbing power of thesurrounding air is at its maximum when the wet and dry bulb temperaturesare at their highest (the relative humidity remaining substantiallyconstant),

- the dr in ower of the air havin no ill effect at this period becauseof the high degree of wet temperature. The drying power of the air thenfalls rapidly so as to discontinue the rapid drying of the surface ofthe lumber and thereby avoiding case hardening during the time thedifferential pressure within the lumber is acting to force the moistureoutwardly from the core. Thus an equilibrium is established between theinterior and exterior of the lumber with no i l effects to the latter,before the next cycle of rapid surface drying is started.

The invention claimed is:

1. The process of rapidly drying lumber or the like consisting insubjecting the same to a series of cycles of treatment, each cycleconsisting of subjecting the lumber to a simultaneous and correspondingrise in dry and wet bulb temperature followed by a simultaneous andcorresponding decrease in the dry and wet bulb temperatures, therelative increase and decrease of said dry and wet bulb temperatures foreach cycle being substantially the same.

2. The process of rapidly drying lumber or the like consisting inmaintaining a substantially constant relative humidity by intermittentlyand simultaneously raising and lowering the wet and dry bulbtemperatures, the relative increase and decrease of said dry and wetbulb temperatures for each cycle of treatment being substantially thesame, thereby maintaining a substantially constant differentialtherebetween.

3. The process of rapidly drying lumber or the like consisting inexposing the lumber to the air in a kiln, and simultaneously, repeatedlyand alternately, raising and lowering the dry and wet bulb temperaturewithin said kiln between wide limits, the relative raising and loweringof the wet and dry bulb temperatures for each cycle of treatment beingsubstantially the same to maintain a substantially constant relativehumidity.

4. The process of rapidly drying lumber or the like consisting inexposing the lumber to the air in a kiln, periodically and repeatedlyraising the dry bulb temperature and im mediately lowering the same, andsimultaneously raising and lowering the wet bulb temperature therewith,the relative increase and decrease of said dry and wet bulb temperaturesfor each cycle of treatment being substantially the same, to therebymaintain a substantially constant relative humidity.

5. The process of rapidly drying lumber or the like consisting inexposing the lumber to the air in a kiln and subjecting the same to anumber of successive cycles of treatment, each cycle consisting ofsimultaneously, correspondingly and immediately raising and lowering thedry and wet bulb temperature within said kiln, the relative increase anddecrease of said dry and wet bulb temperatures of each cycle beingsubstantially the same, to maintain a substantially constant relativehumidity.

6. The process of rapidly drying lumber or the like consisting inexposing the lumber to the air in a kiln, and subjecting the same to anumber of successive cycles of treatment, each cycle consisting ofsuccessively increasing and diminishing the wet and dry bulbtemperatures in said kiln, the relative increase and decrease in saidtemperatures of each cycle remaining approximately the same, whereby toproduce a substantially constant relative humidity in said kiln.

In witness whereof, I have hereunto affixed my signature.

FREDERICK W. RUBIN.

